Pension and Other Postretirement Benefits - Components of Pension Expense for Defined Benefit Pension Plan (Detail) - Pension Plan, Defined Benefit [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Service cost - benefits earned during the period	$ 4	$ 5	$ 5
Interest cost on projected benefit obligation	49	51	50
Expected return on assets	(63)	(62)	(61)
Amortization of net actuarial loss	27	24	35
Net periodic postretirement benefit cost	$ 17	$ 18	$ 29